1. Field of the Invention
The present invention is in the field of methods and apparatus used to prevent the presence of paramagnetic or ferromagnetic objects near a magnetic resonance imaging (MRI) system.
2. Background Art
Paramagnetic and ferromagnetic objects are highly unsafe near MRI systems, because the strong magnetic gradients caused by MRI magnets exert a strong force on such objects, potentially turning them into dangerous missiles. Several accidents, some fatal, are known to have occurred as the result of someone inadvertently carrying such an object into the MRI room. Current MRI safety practices rely on signage and training to prevent people from taking such objects into the MRI chamber. There is a need for a technical means to prevent the accidental transportation of such objects into the MRI chamber, or to warn of such an occurrence.
Use of conventional metal detectors, whether portals or wands, would not be efficient for this purpose, because they do not distinguish between magnetic and non-magnetic objects, and only magnetic objects are dangerous. Conventional systems generate an audio-band oscillating or pulsed magnetic field with which they illuminate the subject. The time-varying field induces electrical eddy currents in metallic objects. It is these eddy currents which are detected by the system, to reveal the presence of the metallic objects. There is no discrimination between ferromagnetic objects, which are dangerous near an MRI system, and non-magnetic objects, which are not. As a result, conventional systems would generate far too many false alarms to be usable in this application. The invention described herein solves the problem by detecting only paramagnetic and ferromagnetic objects, which are exactly those that must be excluded from the MRI room.
The present invention specifically addresses a need for intra-operative application of MRI. The current surgical technique for most major cancer surgical procedures is for the surgeon to remove the tumor as delineated by the preoperative MRI. Upon conclusion of what is believed to be adequate tumor excision, the skin incision is sutured, and the patient is sent to recovery. A postoperative MRI is then performed in the imaging center, and, it is hoped, no residual tumor is found.
Unfortunately, residual tumor is commonly present. Neurosurgery is a field in which the use of MRI is ubiquitous. With conventional neurosurgery, in an effort to make certain that “all the cancer” is removed, neurosurgeons often excise significant sections of normal brain tissue, recognizing that it is difficult to determine visually the exact borders of the cancer, even with the use of an operating microscope.
When the cancer is incompletely excised during the initial neurosurgical procedure, however, as documented by postoperative MRI after the patient is sent to recovery, the patient is returned to the operating room for further surgery. This is often on a different day than the first surgical procedure, causing tremendous anxiety to the patient and to the patient's family.
The patient must be re-anaesthetized, the craniotomy flap reopened, and more brain tissue excised, in the hope that the tumor has been completely extirpated. A postoperative MRI is obtained to confirm or deny this.
The use of intra-operative MRI can change the hit-or-miss scenario described above, and a number of centers have established intra-operative MRI facilities. The advantage of intra-operative MRI is similar in concept to that of Moh's procedure for skin cancers, in which sequential intra-operative biopsies of tumor margins are examined by a pathologist and determined to be cancer-free before the surgery is concluded and the incision is sutured. The difference is that with intra-operative MRI, confirmation of the tumor margin is made not by pathological microscopic examination of excised tissue, but by magnetic resonance imaging.
Two competing methodologies are becoming available to accommodate intra-operative MRI. In the first methodology, the MRI magnet is brought into the operating room on a mechanized retractable transport tube. The operating room table remains stationary, and intra-operative MRI is performed (“mountain-to-patient strategy”). After intra-operative MR imaging, the transport tube is retracted into the adjacent room, and the surgery continues, based upon information gleaned from the intra-operative MRI. This procedure can be repeated until the surgeon and his or her team are completely satisfied that the tumor has been completely eliminated.
The second methodology involves bringing the patient to the MRI magnet, which is in a separate dedicated ante-room. In this instance, the MRI suite can be used for other intra-operative MRI procedures, as the ante-room can be fed, as it were, by one or more operating room theatres. This second (“patient-to-mountain”) strategy requires that the OR table and the anesthesia cart to which the patient is obligatorily attached be transported into the magnet room.
In either strategy, it would be helpful to be able to thoroughly scan the patient, and the operating room table and anesthesia cart for ferromagnetic threat objects. To minimize the missile threat in the intra-operative scenario described herein, the use of very sensitive ferromagnetic-only detection portals may well prove beneficial.
A major problem at this time, however, is that even so-called “MRI Safe” pieces of equipment, such as designated gurneys, anesthesia carts, and the like, often contain small ferromagnetic components, such as wheel bearings, or gauge displays on monitoring equipment. Because of the very large mass of the gurney or cart relative to the very small amount of ferromagnetic materials present, however, the gurney or cart is not propelled toward the MRI magnet. Nevertheless, the ferromagnetic material will trigger a false-positive alarm when the gurney or cart passes through a sensitive ferromagnetic detection portal. This alarm response is baffling to the surgeon, as he or she cannot know whether the alarm emanates from the gurney or cart, or from a true ferromagnetic threat with the potential for causing harm.
A worrisome threat is a ferromagnetic hemostat clamp deep within the abdomen of a patient undergoing cancer surgery. If the surgeon is unaware of the presence of this clamp, and intra-operative MRI proceeds, the magnetic field of the MRI instrument could cause the clamp to tear through the patient's tissues, causing hemorrhage, nerve damage, and other catastrophically untoward consequences.
Many surgical instruments are non-magnetic and non-magnetizable, but not all. Of course, with intra-operative surgery employing MRI, a great effort should be made to have all instruments in the surgical environment be non-magnetic and non-magnetizable, but this is not always practical.
What is required, then, is a ferromagnetic detection system which detects these ferromagnetic instruments before the intra-operative MRI procedure. The present invention provides an apparatus and method for achieving this requirement. In addition to intra-operative MRI, the present invention can be employed for the detection of foreign objects within a patient prior to conventional MRI. It can also detect ferromagnetic threat objects on the person of a subject, such as bobby pins or nail clippers.